Amplifier



Oct. 28, 1941 s-n w 2,260,493

AMPLIFIER Filed Nov. 1; 1940 FIG.

l0 2 q E \g INVENTOR A. L. ST/LLWELL 7- W @241 A TTORNEL .fied is other than ground potential.

Patented Oct. 28, 1941 AMPLIFIER Albert L. Stillwell, Westfield, N. .L, assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 1, 1940, Serial No. 363,803

Claims.

This invention relates to electric wave amplification and more particularly to electric wave amplifiers comprising amplifying discharge'devices having indirectly heated cathodes.

In certain types of electric wave amplifiers proposed heretofore the cathode in one or more of the amplifying tubes employed is maintained at a potential that for frequencies to be ampli- If insuch case the tubes are of the so-called heater type comprising a heater element within the envelope of the tube and in proximity to the cathode, the capacitance that inherently exists between the cathode and its heater may deleteriously affect the operation .of the amplifier. Thus, in-a'negative feedback amplifier of the cathode feedback .heater capacitances mentioned will often be found highly undesirable.

A principal object of thepresent invention is to minimize the effect of cathode-to-heater ca- .pacitance on the performance of amplifiers of the kind described.

In accordance-with a preferred embodiment of the invention, the foregoing object and others that will appear hereinafter are achieved by associating with the cathode heater circuits, impedance elements ,or networks of such character and disposed in such relation to the deleterious capacitance that the latter, although not reduced in magnitude, is effectively isolated from the signaling circuit. The nature of the invention will appear more fully from the following description of the preferred embodiment illustrated in the accompanyingdrawing. In the drawing,

.Fig. .1 .illustrates schematically a threeestage negative feedback amplifier embodying the invention inpreferred form; and

Figs. 2 .and 3 are simplified equivalent circuit diagrams to which reference will be made in the course of the description of Fig. 1.

The amplifier depicted in Fig. 1 .is a three-stage negative feedback amplifier .of .the cathode feedback type. as disclosed more .fully in the patent power supply transformer l5.

provide ground for high frequency currents. .cathode heaters l ,and.9 were connected in the to West cited hereinabove. As will be apparent to those skilled in the art many details not essential to an understanding of the present invention have been omitted, and for such details reference may be made to the West patent. The specific amplifier shown comprises three tubes l, 2 and 3, connected in tandem by coupling impedances Z1, and the amplifier as a whole is adaptedfor connection to an external circuit by means of input transformer 10 and output transformer II. The cathode 5 of the intermediate amplifier tube 2 is connected to ground. Cathodes 4 and 6 of tubes I and3, respectively, are connected together to one terminal of an impedance network Z2, the other terminal of which is connected toground. The anode circuits of tubes 1 .and 3 are completed to their respective cathodes 4.and 6 through the common impedance element Z2, and the resultant voltage appearing across the latter is applied through the secondary winding of input transformer 10 to the control grid of .tube I to effect gain-reducing feedback.

The impedance Z2 will be recognized as being a feedback coupling impedance connected across a beta or feedback circuit. An impedance Z3 may be inserted in the lead to cathode 6 to provide additional local feedback for tube 3. It may be supposed for specific example that the amplifier of Fig. 1 is so proportioned as to be adapted for the amplification 0f multiplex carrier telephone or carrier television signals rangingin frequency from kilocycles to 3 or 4 thousand kilocycles per. second.

Cathodes 4, 5 and .6 are associated with respective cathode heaters "l, 8 and 9 which are connected directly or indirectly to-a 60-cycle or other suitable heating current source. Cathode heater 8 is indicated as being connected in the usual manner to the secondary Winding of a Cathode heaters 1 and 9 on the other hand are indicated as being connected to the power supply transformer through an impedance network 20, the nature and purpose of which will appear presently. The transformer end of a cathode heater circuit is almost always, if not quite invariably, at ground potentialin sofar as radio frequency currents are concerned. Often the mid-point of the heater transformer secondary winding is directly grounded; sometimes as in Fig. 1 the transformer secondary terminals are capacitively shunted to ground; and in any case the mass andstray capacitances of the power transformer effectively usual manner to the heater current source, that is, directly as heater 8 is connected, it is evident that the capacitance (24-1 between cathode 4 and heater 1 would directly connect cathode 4 capacitively to ground and that the heater-tocathode capacitance 06-9 in tube 3 would similarly capacitively connect cathode 6 t ground.

In the circumstances assumed, the two cathode-to-heater capacitances are related to the feedback impedance elements in the manner shown diagrammatically in Fig. 2. That is, ca-

pacitance C44 constitutes a direct shunt across the beta circuit coupling impedance Z2 and ca pacitance C64 effectively shunts the series combination of impedances Z2 and Z3. Inasmuch as impedance Z2 would ordinarily'be of small magnitude as compared With impedance Z3, the capacitance Cs-s has the principal effect of capacitively shunting impedance Z3. ing capacitance in each case introduces an inherent limitation on the design and performance of the amplifier. The shunting of impedanceZs which is assumed to be of high impedance, moreover, makes more critical the proportioning of ,the internal elements thereof inasmuch as the total capacitance across Z3 would ordinarily be strictly limited. It may be noted that the capacitances indicated are respectively proportional in size to the number of amplifier tubes operated in multiple, and that if local feedback in thelast stage were dispensed with, all of the capacitances would be lumped across the beta circuit impedance Z2.

The undesirable condition indicated in Fig. 2 is avoided and other advantages are secured in accordance with the invention by interposing between cathode heaters and 9 and the grounded or effectively grounded end of the heater circuit a network 20 that offers high longitudinal impedance to the flow of current in the upper portion of the frequency range, that is, high impedance at all frequencies for which the oathode-to-heater capacitive reactance would be significant. The specific network shown is of H configuration, the shunt arm of which comprises two series connected resistors 2| grounded at their junction. The two series arms adjacent theheaters comprise each a series inductance 22 shunted by a resistor 23. The two series arms adjacent the transformer l5 comprise series inductances 24.

In a particular case in practice resistors 2| were 000 ohms each; elements 22, 100 microhenries each; resistors 23, 6000 ohms; and ele- 'ments 24, 1 millihenry each. The several rewill be understood that the network 20 interposes,

in the heater lead a longitudinal impedance that is many times as large as Z2 at allsignificant frequencies. The impedance offered by network 20 to the flow of heating current is practically negligible. To avoid series resonating of inductances 24 and the stray capacitance of the power feedback impedance Z3, capacitance Cs-9 is now its associated heater.

transformer, at least within the signal frequency range, the stray capacitance is augmented by the large parallel capacitance afforded by by-' pass condensers 25.

Fig. 3 shows in diagrammatic form the changes brought about by the insertion of the longitudinal impedance Z; of network 20. It will be seen that the capacitance 04-1 no longer di- .rectly shunts the beta circuit impedance Z2, for

the high impedance Z4 is now interposed in the shunting connection. Capacitance Cs-9 is similarly isolated from shunting effect on impedance With respect to its shunting effect on local 'connectediri series with capacitance C44 across impedance Z3, the effective shunting capacitance thereby being substantially reduced. In this way the asymptotic performance of the feedback path can be materially improved and hence also the feedback in the band.

A feature of the invention is the-protection it affords against complete disability of the amplifier in the event of an accidental short circuit arising between a cathode, such as 4 or 6, and From Fig. 2 it is evident that if the capacitance 04-7 were converted into a conductive short circuit, as by contacting of cathode 4 and heater 1, the beta circuit of the amplifier would be directly short-circuited by the fault. Fig. 3 shows on the other hand that with an arrangement in accordance with the invention, would still leave a high impedance Z4 in the shunting connection and the operation of the amplifier would be not substantially affected.

Similar remarks may be made with reference to impedance Z2 and the possible short-circuit of capacitance Cs 9. Although 3 the present invention has been described principally with reference to the illustrated preferred embodiment, it will be evident to those skilled in the art that the invention is susceptible of application in other forms within "the spirit and scope of the appended claims.

What'is claimed is: -1. A signal amplifier of the cathode feedback type comprising amplifier tubes of the cathode heater type, a heatin current source connected to the heaters of said tubes, the cathode of at least one of said tubes being maintained at high potential .for signal frequency, and an impedance network interposed in the heating current leads associated with the said cathodes that are maintained at high potential forsignal frequency, said network introducing high longitudinal impedance to ground for at least signal frequencies in the upper portion of the signal frequency range.

2. A signal amplifier of the cathode feedback type comprising at least three'stages of amplification connected for cathode feedback operation,

a signal amplifier tube of the cathode heater 'typein each of said stages, a local feedback impedance element in the cathode lead of the tube in the third of said stages, an alternating current source connected to the cathode heaters of all of said tubes, and an impedance network in-' terposed between the first and last stage heaters and the point of connection of the second stage heater to said source, said network offering low impedance to the heating current and relatively high impedance to signal frequencies.

3. An electric wave amplifier of the feedback type employing heater type tubes, said amplifier having a beta-circuit such that at least "one "of short-circuiting of capacitance 04-7 the cathodes of said tubes is maintained off ground potential for all frequencies in the frequency range to be amplified, and means in the heater lead associated with said cathode offering substantial longitudinal impedance to all frequencies in the amplified range, whereby said beta circuit is protected against disability in the event of a short circuit between said cathodes and their associated heaters.

4. An electric Wave amplifier of the feedback type, said amplifier having a beta circuit and comprising heater type tubes, the cathode of at least one of said tubes being connected to a point in said beta circuit of high potential relative to ground for frequencies to be amplified, whereby the capacitance between the said cathode and its associated heater tends to shunt said beta circuit,

and means in the heating current circuit to said heater introducing such impedance that the shunting efiect of said capacitance is substantially reduced.

5. An electric Wave amplifier for the amplification of high frequency signals, comprising signal amplifier tubes of the cathode heater type, at least one of said cathodes being connected to ground only through an impedance that is high at least for frequencies in the upper portion of the signaling frequency band whereby cathodeto-heater-to-ground capacitance tends to reduce the said impedance, and impedance means in the heater circuit associated with said cathode for increasing the impedance to ground for said signal frequencies.

ALBERT L. S'I'ILLWELL. 

